The booster may be pneumatic and comprise a rigid casing divided into two pneumatic chambers by a moving partition which may be subjected to a difference in pressure between the chambers under the action of a valve actuated by the manual-control member.
A braking device of this type is known, for example from EP-B-0 662 894.
The conditions under which braking is performed may differ. A first scenario corresponds to ordinary conditions when an obstacle is seen from a distance and braking is performed relatively gently; this braking is known as “slow braking”. Another scenario is that of abrupt braking or “emergency braking”, for example when an obstacle appears suddenly in front of the driver who has to bring his vehicle to rest as quickly as possible.
In order to satisfy these various braking conditions, a boosted braking device of the above defined type has been supplemented by an emergency assist valve capable of initiating the intervention of at least two boost ratios corresponding respectively to slow braking and to emergency braking; the boost ratio for slow braking is lower and the reaction opposing the advance of the manual-control member is greater. For emergency braking, the boost ratio is higher and the reaction against the manual-control member is weaker, which means that the driver can brake for longer and more heavily.
By convention, the term “front” will be used in the remainder of the description to denote a direction directed from the control member toward the master cylinder and the term “rear” or “back” will be used for the opposite direction.
The emergency assist valve comprises a reaction piston which slides in a sealed manner at the rear of a bore of the primary piston, the front part of this bore communicating with the interior volume of the master cylinder, the reaction piston comprising an axial passage through which there passes a ratio control which at its front end has a head located in a chamber lying between the reaction piston and a bushing housed further forward in the bore of the primary piston, this bushing having an axial passage in which a rapid piston of cross section smaller than that of the reaction piston can slide, which rapid piston also has an axial passage open at each end, the rear end of the rapid piston and the head of the ratio control forming a valve capable of shutting off the inlet to the passage of the rapid piston.
The slow boost ratio involves the reaction piston, which has the larger cross section; emergency braking involves the rapid piston which has a smaller cross section.
It has become apparent that such a braking device equipped with an emergency assist valve, while being entirely satisfactory from the operational and braking force point of view, makes noise, particularly at the end of slow braking.
The object of the invention is, above all, to provide a braking device of the type defined previously, equipped with an emergency assist valve. In which the aforementioned drawback does not occur, or at least occurs to a lesser extent. It is also desirable for the solution proposed to remain simple and economical.